On Duolingo, there are issues in some of the pronunciations. In my experience google translate often tends to give an acceptable approximation. Another particularly useful source is duden.de.
As for the difference, they are two completely different words that sound nothing similar. 'Kurz' has a noticeable z ending while 'gross' is simply pronounced as in Englisch.
Edit: you can also use https://de.pons.com/ to help with the sounds